The cooking fat changes as you move across India, and the change is abrupt rather than gradual. What each region used was determined by agriculture, and the cuisine then formed around it.
Agriculture set the starting point
Coconut grows abundantly along the southern coasts, mustard across the northern and eastern plains, groundnut in the west, and dairy farming supported ghee wherever cattle were kept in numbers.
Before long-distance food transport, a kitchen used what was produced within reach, and the price difference between local and distant fats was decisive rather than marginal.
That constraint lasted long enough for entire regional repertoires to develop assuming one particular fat.
Each fat carries a different flavour load
Cold-pressed mustard oil is sharply pungent from compounds released when the seed is crushed, and heating it to smoking point mellows that pungency into something rounder.
Coconut oil is sweet and distinctly aromatic, and it sets solid at cool temperatures, which affects how a dish tastes as it cools on the plate.
Ghee carries the browned dairy notes created during clarification, and groundnut oil is comparatively neutral, which is why it spread furthest as a general cooking fat.
Heat behaviour differs as much as taste
Unrefined oils contain plant particles and free fatty acids that break down at lower temperatures, which is why traditional methods heat mustard oil hard before adding anything.
Ghee tolerates high heat because the water and milk solids that would burn have been removed, making it suited to frying and to long tempering.
Coconut oil sits between them and is generally used at moderate heat, which fits the southern pattern of tempering briefly and adding to a dish rather than frying at length.
Why substitution changes the dish
A recipe developed around mustard oil is usually balanced against its pungency, so replacing it with a neutral oil leaves the other seasonings unopposed and the dish tasting flat.
The reverse is equally true, and mustard oil in a preparation designed for coconut will dominate everything around it.
Cooks who need to substitute generally adjust the spicing at the same time rather than swapping the fat alone.
How the pattern is shifting
Refined blended oils have spread widely because they are cheap, stable and neutral, and a great deal of everyday cooking across all regions now uses them.
The traditional fats persist most strongly in dishes where the fat is part of the identity, so a festival preparation may still specify ghee while daily cooking does not.
That split means the regional distinction survives in the dishes people describe as characteristic rather than in the food cooked most often.